Being someone quite interested in batteries, something I've been doing both professionally and for fun, I tend to have battery run everything. Lawn mower, car (hybrid), edge trimmer and ofcourse e-assist cargo bike.

The battery on the bike is a cheap (for its capacity) 36V 15Ah pack, hasn't failed me yet but I've used it for about 5000km of riding and before it fails me many km away from home it is wise to know what I can get out of it.

Choosing the load to put on a battery often determines what amp hours (Ah) you get out of a battery. For Lithium Ion and Lithium Polymer (this battery) a general load is 0.2 of rated capacity (0.2C). I chose a load of 2.3A, not quite 0.2C but this was done due to what are often over-rated values from Chinese suppliers, you know, like those kazillion lumen lights you get.

Attached is the report I get from the equipment I use.

This pack is made up of a 10S2P arrangement, that is, 2 cells are wired in parallel and each of these are in series. 3.6V per cell pair x 10 gets you 36V. Cells are depleted at 3V per cell, fully charged at 4.2V per cell. So you would expect the pack to go down to 30V when flat.

Shock and horror (see test), the pack basically turned itself off when the voltage only reached 35.8V. At first I thought maybe I got the type of pack that turns itself off after several hours of not much current draw so I reset the battery and started the test again, see the green curve. Again the battery turned off at about 35.8V.

I know I've run this pack down to 32V in the distant past and there was no cutoff so what is happening this time?

Lithium cells are finicky, voltage too high and you could cause a fire, get them too low and you could damage the cells which when you try and charge next time could rupture and again cause a fire. For this reason manufacturers MUST put protection circuits in place. These essentially cut off the output if going too low in voltage or overcharge. Each cell is monitored for under or over voltages.

Experience tells me that at least one of the cells in my 10 series pack is too low causing a premature cutoff.

I hope I'm not boring you, if you've read this far, thanks for sticking with it.

I opened up the pack and measured each cell, yep, one cell pair was lower in voltage than the rest. This one was cutting off the output of the whole pack.

I'm carefully charging this one cell pair now. Once it is up to the same voltage as the rest I'll run another test.

Crossed fingers.

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I opened up the pack and measured each cell,...

Any chance of posting pics of the opened up pack to show how the cells are arranged , and is opening it up a dangerous process or are each cell effectively packaged and isolated like a dry cell battery? 

I hope you get the cell rebalanced and save the pack. I thought the BMS (battery management system) was designed to rebalance the pack if you leave it plugged in over night, which I try and do once per week. If you need a new battery I can highly recommend this place http://em3ev.com/store/index.php?route=product/category&path=35 I've got two from this guy now and he does great packs.

I hope I can as well. Not all battery packs have battery balance circuits in them, part of the difference between a good pack and a cheap pack. Some balance only during charge, others during discharge, some only when a pack is active in a system.

There are a lot of restrictions on shipping lithium packs, must be expensive to get here.

He uses a battery friendly shipper (FedEx in 5 days) so for a Samsung 12amp hour 36volt pack, it was $540 delivered. Dearer than many, but he has a good reputation, at least on line.

after discharging the pack to the cutoff point again I decided to discharge each cell to 3V to see what capacity they has left and positively identify the soft ones.

Cell 5 (see attached), starts low and ends quickly. This is the dud pair of cells. Since this cell voltage goes higher upon charge and lower on discharge than the rest I'd say they are a bit soft. Bugger, usually means they are stuffed.

The maximum capacity of the pack is now only as good as this cell, enough to get me home and work but I'll have to be wary on longer rides.

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That's a shame, but it sounds like it's had a good innings. You sound unafraid of taking you batteries apart (I have once crossed the positive and negative accidentally and the ensuing fireworks and vaporisation of my multimeter probe tips put me off doing too much more). Maybe you could get a suitable BMS and convert the pack to a lower voltage, my Yuba runs on 24 volts, but would likely be happy up to 30 volts plus I recon. http://www.bmsbattery.com sell 24 volt hub motors too.

No I'm not afraid and yes you have to be careful. I've been doing this for a long time, designed and was selling one of the first gas gauge packs and compatible chargers to Australia's largest battery importer.

I have plenty of BMS on hand and will likely make myself a lower voltage pack as you suggest. For now though the pack will still be useful for transport

I'll probably make a nwe pack using some Sanyo cyclindrical cells welded up into a 36V 15Ah string. Lower weight and custom configuration to fit the bike better.

I personally like supporting Australian companies as much as I can, being one myself. http://www.zeva.com.au/ have some good gear and their data sheets are written in a straight, no bullshit manner.

Is there a reason why you don't just replace the dud cells?

On another note I am riding a BH Emotion Cross (Samsung battery pack) it is now 1yr old and I can't really feel a difference in the battery capacity (i.e. it still goes pretty hard). BH says the battery pack is good for 30,000KM !! I'll be happy to get half that.

Replacing the dud cells will mean I'll have a good cells mixed with aged cells. Not a good idea as some will go flat earlier than others and teh pack will cut off. Upon charging the new cells will not fully charge and the whole pack becomes unbalanced.

I can get 30000km out of this pack as well, but range will be forever decreasing. ATM I would probably get 45km range from it the way I ride. If i back off a bit more often I'll get a much greater range. My commute is 35km on the way home, I have a while before this pack is useless for the style of riding I have on the way home. When it starts cutting out at 35km I'll back off the power and continue commuting at a slower pace.

I understand you are going to you going to make a new battery at some stage so this is just hypothetical now, would you be better off with replacing the dud cells with new cells (and the new cells not fully charging) but returning your battery pack to a better capacity than existing dud cells reducing the overall capacity?

I guess the question is, which is likely to do more damage, dud cells failing completely or new cells doing something else?

well yes I could repalce the dud cell and get a better pack, up to the next weakest cell.

I have to balance the investment in time replacing the cells and the eventual aggregate of time wasted if I have a slower trip home plus the risk of the dud cell getting more dudderer very quickly.

I'm on the side of making a whole new fancy pack and using the OK cells from this pack to make a lower voltage but very high amp hour battery for my off grid solar system.

for my off grid solar system. Wow, Rob you are just so behind ...

Pilot project: Netherlands build first solar cycle track in the world

http://www.spiegel.de/reise/aktuell/solarradweg-in-den-niederlanden...

The community Krommenie, 25 kilometers from Amsterdam currently receives, a pioneering high-tech bike path. The pilot project with solar cells covering is first extended 70 meters long and up to 2016 on 100 meters. The construction work of the consortium SolaRoad last until October 24, Opening Day is celebrated on 12 November.

To realize the project, the province of North Holland with a one and a half million Euros most of the cost controls in. Overall, the total expenditure amounts to three million euros. With its 100 meter length of the cycle track to produce as much energy in order three households with electricity to.

Roads can produce energy for cars and e-bikes

On the site of SolaRoad states that the project is "unique". "Convert sunlight into electricity over the road surface." Sten de Wit, one of the project of SolaRoad, is already planning for the future: "So roads could be producing electricity for the electric cars that run on these." Also e-bikes could be charged on the road itself.

And how does it work? The bike path consists of rectangular concrete modules , in which there are solar cells. Furthermore, there is a one-inch-thick layer of glass, the paved surface. It is translucent and very stable so that it can withstand the weight of a truck.

The road is expected to produce 30 percent less energy than a plant on a rooftop loud SolaRoad. Solar panels on the roof generate more power because they can be aligned with the sun.

The idea of ​​using transport routes as power plants is not new. An American inventor-couple has two million dollars in the past few months via crowdfunding collected for the construction of the first solar road in the world . Now the Dutch are the Americans Julie and Scott Brusaw preempted. The couple plans to go with their "Solarroadways" in series production.

The Brusaws have calculated: If every highway in the United States Solar street, the United States could produce three times as much electricity as the country currently consumes. However, solar street is also three times as expensive as conventional.

SolaRoad from JM Duurzame Communicatie on Vimeo.

So Dutch roads produce 30% less electricity than roof top solar cells, but Australian roads produce nothing but crap.

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